In lighting solutions, LEDs (light-emitting diodes) are playing an ever greater role, made possible by the advances in LED technology in recent years. LED lighting arrangements can be designed to emit white light, necessary for indoor and outdoor illumination purposes, by combining red, green and blue LEDs in solid-state lighting (SSL) solutions. Some LEDs can be coated with phosphor to convert the emitted light into another colour, for example blue ‘pump’ light can be converted into yellow, green or red light. Such coated LEDs can be combined with non-coated LEDs in an arrangement to give white light. Typically, phosphor-converted white-emitting LEDs are obtained by a combination of phosphor-converted yellowish light and some part of the blue pump light. The development of LEDs with a high light output allows these to be used to replace the comparatively inefficient incandescent light bulbs, which are being phased out. High-power LEDs currently available can produce up to several hundreds of lumens while consuming much less power than conventional incandescent bulbs. For example, the Luxeon Rebel achieves a luminous efficacy of more than 100 lm/W.
The total light output of an LED arrangement depends on the number of LEDs used and the power of the individual LEDs. Since LEDs are semiconductor devices, they are easily combined on a common substrate in a chip package. Present-day LED chips for lighting purposes comprise a number of ‘strings’ of serially connected LEDs. The number of LEDs in a single string is chosen so that the sum of the forward voltages of the LEDs approximately equals the desired voltage drop across the entire string. Such LED chips can in turn be grouped and mounted onto a light-fitting.
A conventional LED requires a low voltage (in the order of 5 V) and a direct current (DC), whereas mains electricity is high voltage (220V in Europe or 110 V in the USA) and alternating current (AC). To drive conventional LEDs using mains power, full-wave rectification and transformation must be performed to obtain the necessary low-voltage DC signal.
In an alternative approach, an AC-LED chip may be used, i.e. a chip incorporating one or more LEDs and designed specifically to be driven directly using an AC voltage.
Here, the term ‘LED’ can refer to a light-emitting semiconductor junction, but also to a packaged light-emitting device comprising multiple such junctions. This type of LED does not require a DC converter. An AC-LED chip essentially comprises two strings of series-connected LEDs, connected anti-parallel or inverse-parallel, typically at die level or via bond-wiring of several dies, so that one string is active (emitting light) during a positive half of the current cycle, while the other string is active during the negative half. The semiconductor die is designed so that the forward voltage of each string is approximately equal to the root-mean-square (rms) value of the mains voltage from which the chip is to be driven, and a simple ballast circuitry can be used to limit the current. This ‘bipolar’ structure gives an integrated reverse polarity protection as well as electrostatic discharge protection. Such AC-LED chips (or simply “AC-LEDs”) are becoming interesting for low-cost general illumination. However, the light produced by AC-LEDs driven from the AC mains supply can exhibit an unacceptably high degree of optical flicker, caused by the rapid alteration in polarity at mains frequency. This flicker can be irritating, particularly in the case of indoor lighting applications.
In one approach to this problem, an existing AC-LED chip can be driven instead with a DC current. In such a solution, the AC mains input is smoothed, current limited and surge protected to obtain the required DC signal. The AC-LED chip can be directly connected to this DC signal and driven at a fixed polarity, giving an improved light quality and efficiency of conversion of electrical energy to light. However, in this mode of operation, only one part of the AC-LED chip is continually driven with a forward current, while the other part is continually exposed to a reverse bias voltage and is effectively not used. Assuming the strings comprise essentially equal numbers of LEDs, only 50% of the chip is used to produce light when driven using this method. Apart from the poor utilization, this mode of operation leads to a reduction in lifetime of the AC-LED chip, because, when driven continually with a DC signal, only one of the two strings of LEDs is continually ‘stressed’ with a drive signal to generate light. The phosphor material used to convert the emitted light is therefore also always ‘stressed’ in this active string, and will degrade over time more quickly than in an AC-LED, which is driven with an AC drive signal and in which both strings are driven alternately.